53.
Two Separate Populations. Short-period comets might be explained if comets formed near the giant planets. However, this would not produce the number of needed near-parabolic comets. The average comet flung out toward an Oort cloud, but not expelled from the solar system, would end up far short of where the Oort cloud supposedly is.115 [See Figure 166 on page 297.]

54.
Jupiter’s Family. Comets in Jupiter’s family have an average life span of only about 12,000 years. They could not have accumulated over millions of years.

55.
High Loss Rates of Comets. Several locations for cometary nurseries in the giant-planet region have been proposed. Oort favored the asteroid belt, between Mars and Jupiter, if such a nursery was needed to supply the Oort cloud. Later, FernÅ ndez showed that, if comets were born near Jupiter, Jupiter would expel too many from the solar system. To account for today’s high loss rate of comets from an Oort cloud would require 10,000 Earth masses of comets in a Jupiter birthing region 4.5 billion years ago—“too large to consider it dynamically reasonable.”116 Jupiter would have to fling 30 times its mass out to the Oort cloud! No planet’s energy and angular momentum could have done the job.117

FernÅ ndez favored the region between Uranus and Neptune as the place where comets were born and steadily flung out to the Oort cloud. This would require the least amount of cometary birthing material—about 17 Earth masses—or the mass of Neptune. However, Uranus and Neptune would probably not have had the necessary energy and angular momentum.

Overcrowding is another problem. If so many comets began in the giant planet region, they would often collide and fragment. Only about 5% of the comets needed by an Oort cloud could have been delivered to the Oort cloud.118

Ã·pik raised a more serious problem. To form comets in the Uranus-Neptune region and then eject them out to an Oort cloud would require about 100 billion years—20 times the assumed age of the solar system.119

In 1950, Gerard Kuiper (KI-per) theorized that material that almost formed a planet should still exist beyond Neptune, 35–50 AU from the Sun.120 This region, which some believe is filled with comets, is now called the Kuiper belt. Kuiper thought that Pluto expelled the nursery’s comets out to the Oort cloud. Later it was learned that Pluto’s mass was much too small for the job.

Since 1992, ground-based telescopes and the Hubble Space Telescope have detected more than 1,200 large objects in the Kuiper belt, a region that some had hoped was the source of comets in the solar system and in the Oort cloud. Later, it was realized that these objects were ten times too large (25–1,000 miles in diameter) to be comets and too few in number. A reexamination of that region of the sky by the Hubble Space Telescope has failed to detect a comet reservoir.121

59.
Crater Ages. This theory requires a comet nursery containing at least 1013 comets.122 As the giant planets fling some comets out to an Oort cloud, other comets would frequently bombard Earth from close range. The farther back in time, the greater the bombardment rate. As with the original Oort cloud theory, craters from this intense bombardment should be increasingly visible the deeper one looks in Earth’s sedimentary layers. Instead, craters are almost exclusively found in surface layers.